The present invention relates to a method and apparatus for production of rack bars that have a rack along a part of the length of the bars.
Rack bars are used as components of steering equipment for automobiles. The present invention provides a method and apparatus for efficient production of hollow rack bars by plastic working of steel tubes.
Conventionally rack bars for steering equipment have been made from solid bars, however, for the purpose of weight reduction in automobiles, it is desirable to employ hollow materials. FIG. 7 is a perspective view of such a rack bar 1, in which 2 indicates teeth of the rack. A conventional method for production of such a hollowed rack bar is to drill a hole in a solid bar after cutting a rack on the surface. On the other hand, a method for forming a rack by plastic working on a steel tube material is described in Japanese published patent no. 3-5892.
The method consists of two processing stages, i.e. preliminarily flattening of a part of the steel tube, and then forming a rack on the flattened part. For this method, a primary-forming split dies, which can be opened right and left, is prepared to hold a part of a steel tube to be processed, where the split die has a penetrating hole in a part to be formed a rack at a state of the dies being closed. Then, a primary formed piece is obtained by inserting a punch having a flat top into the hole and flattening the part of steel tube. In the next stage, the primary formed piece is held in secondary-forming split dies which can be opened right and left and has an inner shape coinciding with the outer shape of a rack bar product. Namely, the secondary forming split dies have a female pattern of rack teeth in their upper parts at a state of the dies being closed. Then, mandrels are inserted into the steel tube in order to perform ironing of the previously flattened part from the inside of the steel tube. Consequently, the outer surface of the primarily formed piece is bulged, and a rack is formed according to the shape of the secondary-forming split dies.
The above method does not waste material due to the plastic forming method in comparison with the cutting method, and can produce high quality products due to strengthening by plastic working. However, when the method is applied in practice, it has been found that there is room for further improvement because of the rather high cost of the dies. Namely, the primary-forming split dies are long in life because the dies are not subjected to abrasion, while the secondary-forming split dies are relatively short in life because the part of female pattern of the rack teeth is susceptible to being worn out. The rack teeth pattern is not subjected to strong abrasion that deforms the intruded bulging metal, however, because of repeated large stress acting in the rack forming process, the rack teeth pattern can be damaged resulting in the dies failing due to the formation of cracks at the bottoms of the teeth.
Among the dies and tools for use in the above-mentioned methods for production of rack bars, short life because of abrasion in some extent will be allowed in these being subjected to friction between material such as mandrels. However, contrary to the mandrels, since the secondary-forming split dies are complex in shape in the part of the rack teeth pattern and are expensive, short life of the split dies results in high production costs for the rack bars. The present invention is intended to provide an efficient method for production of rack bars by overcoming the above mentioned problems by reducing the cost of the dies and tools.
Namely, the present invention is a method of producing a hollow rack bar by processing a part of the length of a steel tube and forming a rack thereon. The method comprising: holding the steel tube in a set of split dies, which has an inner shape adapted to encircle the whole circumference of the steel tube at the part of the length to be processed and its adjacent parts of both sides except for the dies having a hole coinciding with the part of the steel tube to be processed; inserting a punch into the hole at a closed state of the dies and flattening the part of the steel tube; inserting a rack forming die into a hole of the dies with a same inner shape as aforesaid dies; holding the rack forming die in contact with the flattened part at a closed state of the dies; and inserting mandrels into an inside of the steel tube and forming a rack pattern according to the rack forming die by ironing the flattened part from the inside of the steel tube.
The above method may further comprise inserting a core bar into the steel tube during flattening the steel tube by the punch. Also in the above method, the punch and the rack forming die can be alternatively inserted into the hole of one set of the split dies, for flattening the steel tube and for forming the rack pattern. Otherwise, among two or more sets of the split dies with a same inner shape, one or more sets of the dies can be equipped with the punches for flattening the steel tube, and the other sets of the dies can be equipped with the rack forming dies for forming the rack pattern.
Moreover, the present invention is an apparatus for producing a hollow rack bar by processing a part of the length of a steel tube and forming a rack thereon. The apparatus comprising: one or two sets of split dies, which have an inner shape of encircling a whole circumference of the steel tube at the part of length to be processed and its adjacent parts of both sides except for the dies having a hole coinciding with the part of the steel tube to be processed; a punch with a flat top for inserting into the hole and flattening the steel tube; a rack forming die for holding in contact with the steel tube in the hole of the dies; and mandrels for inserting into an inside of the steel tube and forming a rack pattern according to the rack forming die by ironing the flattened part from the inside of the steel tube. Also in the above apparatus, the rack forming die may have a protruding part that is continuous in both edges at a width direction of the rack.